Useful JsonConverters for serializing with System.Text.Json.

CustomStringConversionJsonConverter.cs

    public class CustomStringConversionJsonConverter<G> : JsonConverter<G>
    {
        private readonly Func<G, string> _convertToString;
        private readonly Func<string, G> _convertFromString;
        public CustomStringConversionJsonConverter(Func<G,string> ConvertToString, Func<string,G> ConvertFromString)
        {
            _convertFromString = ConvertFromString;
            _convertToString = ConvertToString;
        }
        
        public override bool CanConvert(Type typeToConvert)
        {
            return typeof(G).IsAssignableFrom(typeToConvert);
        }

        public override G? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {
            if (reader.TokenType != JsonTokenType.String)
            {
                throw new JsonException();
            }
            var s = reader.GetString();
            return _convertFromString(s);
        }

        public override void Write(Utf8JsonWriter writer, G value, JsonSerializerOptions options)
        {
            writer.WriteStringValue(_convertToString(value));
        }
    }

IgnoreTypeInJsonConverter.cs

    public class IgnoreTypeInJsonConverter<G> : JsonConverter<G> where G: class
    {
        public override bool CanConvert(Type typeToConvert)
        {
            return typeof(G) == typeToConvert;//.IsAssignableFrom(typeToConvert);
        }

        public override G? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {
            if (reader.TokenType == JsonTokenType.StartObject)
            {
                while (reader.Read())
                {
                    if (reader.TokenType == JsonTokenType.EndObject) return default(G);
                }
            }
            else reader.Read();
            return default(G);
        }

        public override void Write(Utf8JsonWriter writer, G value, JsonSerializerOptions options)
        {
            //Do nothing
            writer.WriteNullValue();
        }
    }

PolymorphicJsonConverter.cs

    public class PolymorphicJsonConverter<G> : JsonConverter<G>
    {
        //Inspired by https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/converters-how-to?pivots=dotnet-6-0#support-polymorphic-deserialization

        public override bool CanConvert(Type typeToConvert)
        {
            return typeof(G) == typeToConvert;//.IsAssignableFrom(typeToConvert);
        }

        public override G Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {
            if (reader.TokenType != JsonTokenType.StartObject)
            {
                throw new JsonException();
            }

            reader.Read();
            if (reader.TokenType != JsonTokenType.PropertyName)
            {
                throw new JsonException();
            }

            string? propertyName = reader.GetString();

            if (propertyName != "_type")
            {
                throw new JsonException();
            }

            reader.Read();
            if (reader.TokenType != JsonTokenType.String)
            {
                throw new JsonException();
            }
            string type = reader.GetString();

            //Check type exists
            var tp=Type.GetType(type);

            reader.Read();
            if (reader.TokenType != JsonTokenType.PropertyName)
            {
                throw new JsonException();
            }

            propertyName = reader.GetString();

            if (propertyName != "_object")
            {
                throw new JsonException();
            }

            var rt= (G) JsonSerializer.Deserialize(ref reader, tp, options);

            while (reader.Read())
            {
                if (reader.TokenType == JsonTokenType.EndObject) return rt;
            }
            return rt;
        }

        public static T Cast<T>(object o)
        {
            return (T)o;
        }

        public override void Write(Utf8JsonWriter writer, G value, JsonSerializerOptions options)
        {
            writer.WriteStartObject();
            writer.WriteString("_type", value.GetType().AssemblyQualifiedName);
            writer.WritePropertyName("_object");
            JsonSerializer.Serialize(writer, value, value.GetType());
            writer.WriteEndObject();
        }
    }